Process fluid or gas bearings are now being utilized in an increasing number of diverse applications. These fluid bearings generally comprise two relatively movable elements with a predetermined spacing therebetween filled with a fluid such as air, which, under dynamic conditions forms a supporting wedge sufficient to prevent contact between the two relatively movable elements.
More recently, improved fluid bearings, particularly gas bearings of the hydrodynamic type, have been developed by providing foils in the space between the relatively movable bearing elements. Such foils, which are generally thin sheets of a compliant material, are deflected by the hydrodynamic film forces between adjacent bearing surfaces and the foils thus enhance the hydrodynamic characteristics of the fluid bearings and also provide improved operation under extreme load conditions when normal bearing failure might otherwise occur. Additionally, these foils provide the added advantage of accommodating eccentricity of the relatively movable elements and further provide a cushioning and dampening effect.
The ready availability of relatively clean process fluid or ambient atmosphere as the bearing fluid makes these hydrodynamic, fluid film lubricated, bearings particularly attractive for high speed rotating machinery. While in many cases the hydrodynamic or self-acting fluid bearings provide sufficient load bearing capacity solely from the pressure generated in the fluid film by the relative motion of the two converging surfaces, it is sometimes necessary to externally pressurize the fluid between the bearing surfaces to increase the load carrying capability. While these externally pressurized or hydrostatic fluid bearings do increase the load carrying capacity, they do introduce the requirement for an external source of clean fluid under pressure.
In order to properly position the compliant foils between the relatively movable bearing elements, a number of mounting means have been devised. The most common practice, as exemplified in U.S. Pat. Nos. 3,366,427, 3,375,046 and 3,615,121, is to attach a rod or bar to one end of the foil which can then be retained in a slot or groove in one of the relatively movable bearing elements. Alternately, as exemplified in U.S. Pat. Nos. 3,382,014 and 3,809,433, a plurality of overlapping foils may be individually mounted on a foil base such as by spot welds. The base would then be frictionally held against one of the relatively movable bearing elements. Individual foils may also be fastened directly to one of the movable bearing elements as illustrated in U.S. Pat. No. 4,262,975. Further, a lip or projection at one end of the foil may be restrained in a slot or groove in one of the relatively movable elements. Examples of this type of mounting can be found in U.S. Pat. Nos. 3,511,544, 3,747,997, 3,809,443 and 3,382,014. Individual foils have also been mounted intermediate the ends thereof as described in U.S. Pat. No. 4,178,046.
In order to establish stability of the foils in most of these mounting means, a substantial pre-load is required on the foil, that is, the individual foils must be loaded against the relatively movable bearing element opposed to the bearing element upon which the foils are mounted. It has been conventional to provide separate stiffener elements or underfoils beneath the foil elements to supply this required pre-load as exemplified in U.S. Pat. Nos. 3,893,733 and 4,153,315.
Hybrid bearing support structures for rotating elements, that is, a structure combining two or more different types of bearings, have been known. An example is U.S. Pat. No. 3,951,474 which illustrates a fluid film lubricated compliant foil bearing disposed between two relatively rotatable concentric shafts near the midspan thereof with the ends of the two shafts supported, in one embodiment, by rolling contact bearings. Alternately, a hybrid system may comprise a rolling contact bearing and foil bearing supporting opposite ends of a rotating shaft. While the rolling contact bearing has virtually no radial movement allowable, the foil bearing is comparatively soft and allows for measurable radial movement. Thus, there can be some sway misalignment and hence uneven load distribution which can greatly reduce the load carrying capacity and the life of the foil bearing. There are other bearings situations also which can introduce such misalignment or bearing tilt.